1. Field of the Invention
This invention relates generally to high voltage semiconductor devices. More particularly, this invention relates to high voltage bipolar semiconductor devices in which the base region is surrounded by at least two slots and the collector region is surrounded by at least one slot. The slots are filled with either polysilicon or tungsten. The slots extend beyond the surface of the buried layer or beyond the interface between the epitaxial layer and the substrate.
2. Discussion of the Related Art
The bipolar transistor is an electronic device with two pn junctions in very close proximity. There are three device regions: an emitter region, a base region, and a collector region. The two pn-junctions are known as the emitter-base (EB) junction and the collector-base (CB) junction. Modulation of the current in one pn-junction by means of a change in the bias of the other nearby pn-junction is called bipolar-transistor action. Because the mobility of minority carriers (electrons) in the base region of npn transistors is higher than that of holes in the base of pnp transistors, higher frequency operation and higher speed performances can be obtained with npn devices. For this reason, the following discussion will be in terms of npn transistors but it is to be understood that the discussion is applicable to pnp transistors as well.
The desired device characteristics of bipolar transistors include: high current gain, high frequency ac operation, fast switching speed, high device-breakdown voltages, minimum device size (to achieve high functional density) and high reliability of device operation. In order for high-frequency ac performance and fast switching speed to be achieved, the parasitic resistances of the transistor; R.sub.E, R.sub.B, and R.sub.C, and the parasitic junction capacitances; C.sub.EB, C.sub.CB, and C.sub.CB must be minimized. In addition, high-level injection effects, for example, the Kirk effect should be avoided. For faithful amplification of ac signals, the Early voltage must be high.
To achieve the higher performance required by end users of semiconductor devices, such as computer users or telephony equipment users, it is necessary for the semiconductor manufacturers to provide faster and smaller integrated circuits. There is particular concern for the users of analog devices that typically have a high operating voltage and a requirement for high frequency operation. In order for a semiconductor device to operate at a high voltage and at a high frequency, the doping concentrations of the various pn-junctions must be carefully controlled to provide depletion regions so that the device has the necessary breakdown voltage. As is known in the semiconductor art, there is a tradeoff between high operating voltages and high frequency operation. For example, to obtain a high breakdown voltage, the doping concentrations on each side of a pn-junction should be low, however, a low doping concentration increases the resistance of the device and will decrease the operating frequency. Because the device breakdown voltage is designed to be determined by the base-collector junction because the doping concentrations are lower at the base-collector pn-junction. Therefore, it is necessary to ensure that the device does not breakdown in the sideways direction from the vertical boundaries of the base region to the adjacent structures, which could be either the collector or an isolation structure.
Therefore, what is needed, is a semiconductor device that has a high operating voltage, a high operating frequency and that can be shrunk.